Reference resolution does not understand schema structure #15

Both Walker::parseSchema and Walker::applyConstraints relies on constraints to corrently traverse the schema / instance. Walker::resolveReferences does not and therefore e.g. the following test fails

{
  "title": "issue #15: References under unknown keywords should be ignored",
  "tests": [
    {
      "title": "$ref in unknown property",
      "schema": {
        "unknown": {
          "$ref": "#/foo"
        }
      },
      "valid": [
        {},
        123
      ]
    }
  ]
}

The possible solutions I can think of

1. Add new method to the Constraint interface which would work similarly to Constraint::normalize
2. Merge $ref resolution with the normalize phase, i.e. resolve references during normalize traversal
3. Extract the traversing logic from normalize to a new method, e.g. Constraint::getSubSchemas(). This may require increasing the required PHP version from 5.4 to 5.5 because of yield.

Possible implementations of ItemsConstraint::getSubSchemas.

public function getSubSchemas(stdClass $schema)
{
    if (!isset($schema->items)) {
        // ignore
    } elseif (is_object($schema->items)) {
        yield 'items' => $schema->items;
    } elseif (is_array($schema->items)) {
        foreach ($schema->items as $index => $item) {
            yield "items/$index" => $schema->items[$index];
        }
    }

    if (!isset($schema->additionalItems)) {
        // ignore
    } elseif (is_object($schema->additionalItems)) {
        yield 'additionalItems' => $schema->additionalItems;
    }
}